New Strategies of the Third Century Roman Empire I

‘Posterity, which experienced the fatal effects of his maxims and example, justly considered [Septimius Severus] as the principle author of the decline of the Roman Empire.’

Edward Gibbon, The Decline and Fall of the Roman Empire

Historians have argued for decades that Septimius Severus not only contributed to the decline of the Roman Empire but that he may have been the architect of its downfall. Much of this discussion hinges on his military reforms. Seen over the course of succeeding centuries, his changes in military organisation may have been fundamental in shifting the Roman military might from a strategy of static frontier defence to one of central reserve forces.

It remains uncertain, however, whether Severus himself consciously planned to take the Roman army in this direction or whether he was in fact responding to immediate concerns of defence. The army that emerged one hundred years later would be based on a bipartite military organisation, this would differ significantly from the well-known array of legions that spread out along the empire’s frontiers. The frontier of the fourth century Roman Empire would be settled by limitanei border garrisons, while more centrally located mobile field armies (comitatenses) would be ready to respond to threats from any direction.

Between Severus and this new strategic model, however, lay the battles and upheavals of the third century. Nothing was set in stone and nothing could be taken for granted. Historical hindsight induces a calm complacency, but for those living at the time, Rome (‘the world’) was being beaten back, overrun by adversity and faced imminent destruction. Individuals, not history or destiny, would carve out the empire’s future… if it had one at all.

How the Legions Work

In 192 the defence of the Roman Empire was based around thirty legions, dispersed around the frontiers as needed. The legion was a corporate unit, with its own identity, traditions and battle honours. Its men were often fiercely proud of their legion, a relationship enjoyed today between a British soldier and his regiment.

In size each legion was similar, with a manpower of roughly 5,000 soldiers based around ten cohorts. These cohorts were commanded by senior centurions and were each formed up of six centuries. Despite the misleading title, the century was a combat unit of eighty men led by a highly paid centurion. Cohorts then, being six centuries in size, had a typical strength of 480 men. The cohort and the century were the real tactical units of any Roman force. A cohort may be ordered to ‘follow the flag’ to form a vexillation and join a larger unit needing extra manpower.

The eighty men of a century were billeted in ‘tent parties’ of eight men each, these soldiers would be squad-mates, eating and sleeping together, fighting together, sharing a tent on campaign and a set of twin rooms while in barracks. Centurions had their own staff, not only a servant or two, but also junior officers from the century such as the tesserarius (watch keeper), the signifier (standard bearer and unit treasurer) and the optio (the centurion’s second-in-command). This unit proved quite self-sufficient, its men cooked their own meals and had entrenching tools, tents, arms and armour. It could draw mules from the legion to carry rations, equipment and other baggage and operate independently of its parent unit.

Some, and perhaps all, of the legions elevated the status and responsibility of the first cohort. The writer Vegetius reports that men of the first cohort were the tallest men in the legion.1 Instead of six centuries the first cohort contained only five, although its centuries were kept at double strength (170 men under a single centurion, rather than eighty). This meant that the first cohort became a powerful unit of 800 soldiers, a formation that could be used to spearhead assaults. As the cohort of honour, ‘the first’ was no doubt filled with veterans from across the legion and its five centurions must have been the most senior within the regiment.

In command of a legion was a member of the senatorial order, a legatus legionis. He was a man in his thirties working his way from office to office and was aided by a young senatorial officer, a tribunus laticlavius, perhaps in his late teens or early twenties. He may hope to command a legion himself, later on in his career. Third in command was a seasoned centurion of long service, the praefectus castrorum, or camp prefect, responsible for logistics and administration. Like a senior NCO in any modern army, he will have been able to provide valuable tactical advice to the legionary legate. There were in addition, five young tribunes (tribuni angusticlavii) from Rome’s equestrian class within the legion’s headquarters. Without any specific command responsibility, they were given tasks as and when needed.

Each legion was allocated its own troop of 120 cavalry which carried out scouting, long range patrol, courier duties and screening the flanks of the legion if it was called on to march through hostile territory. Of course the cavalry also had its use in battle but typically this involved mopping up the enemy soldiers after the Roman legionaries had forced them to break and flee.

Additional forces were provided by auxiliary units (auxilia). Whereas legions were only open to Roman citizens (and popular with the poor and landless), the auxiliaries recruited non-citizens from recently conquered provinces. The bellicose tribes that had given the Romans so much trouble during the invasion and takeover of their homelands made ideal army material. Gauls, Germans, Britons, Dacians and others provided men for these auxiliary units.

The relationship between legion and auxilia could be compared to that of the International Security Assistance Forces (ISAF) and the Afghan National Army (ANA) in modern-day Afghanistan. Post-Taliban, the ANA was reformed and underwent training along NATO lines, using US uniforms and equipment. Patrols and assaults in Afghanistan have been conducted by both forces in concert, with ISAF troops taking the lead. Like the Roman auxilia, the ANA often provides support during assaults, but it is also able to conduct its own operations. The analogy cannot be stretched too far, but it gives an idea of the relationship and differing status between the two types of troops. A division of labour, trust and responsibility existed between legion and auxilia.

Auxiliaries really were the junior partner in the military relationship. Pay for an auxiliary infantryman was 100 denarii compared to 300 for a legionary, for example. Auxiliary units were based around a single quingenary cohort (around 480 strong) or a double-sized milliary cohort (around 800 strong). This meant they could be moved easily around the empire as needed but it also pre-empted any troop rebellion. An aggrieved ethnic group now fighting for Rome as an auxiliary unit would have little chance against the higher level command structure and massed cohorts of a Roman legion. Auxiliary uprisings were very rare but did occur from time to time. ISAF troops, wary of individual ANA soldiers within their compounds, may sympathise with Roman legionaries who had no choice but to fight alongside men who had until recently been their sworn enemies …

Most Roman cavalry was provided by mounted auxilia, since many frontier peoples maintained a long tradition of horsemanship. These auxilia were organised as quingenary alae (with a strength of 512 men) or milliary alae (with a strength of 768 men).

Following the Flag

Under the early emperors, a legion or an auxiliary cohort would have many duties, from guarding granaries or post houses, to arresting dissidents, conducting patrols, overseeing some industrial activity and so on. Sometimes these duties took soldiers away from their home fort for weeks or months, but rarely were the troops posted outside of the province.

When an emperor was assembling an army for an assault on some foreign power, or if a frontier defence needed shoring up, he could call on those legions close at hand. He would also need to supplement this force with entire units from much further afield. Legio X Gemina, for example, was based in Germany but ordered to join emperor Trajan’s invasion force for his attack on Dacia in AD 101. It did not return home but was relocated after the Dacian war to Pannonia. This legion was one of three in Pannonia that acclaimed Septimius Severus as emperor in 193. The uprooting of an entire legion to some distant battle-front, along with its staff, families and equipment was the way in which large-scale warfare was waged.

As legions became more entrenched within their home provinces and took up the burden of local frontier administration, it became a difficult matter to lift a legion out of its province. Instead a unit might be ordered to contribute a detachment of men for a particular campaign, a detachment that would return home once the war had ended. When the Jews rebelled against Roman rule in 132, for example, detachments from X Gemina marched east to reinforce the Roman army there. Later in 162, Lucius Verus, the co-emperor of Marcus Aurelius, took a detachment of the Gemina into Parthia, far to the east.

By the time of the Marcomannic Wars, this practice of ordering discrete bodies of men to fight for short periods in wars, before returning them to their forts, had become common. The Latin term for such a detachment was vexillatio, from the word for flag or banner: vexillum. These detachments marched under a temporary Roman military vexillum, which resembled a flag fluttering from a crossbar that was suspended from a central pole. It seems that the vexillations were brigaded together to form a more effective fighting force. They would either return home at war’s end or, as happened to some vexillations, the troops would remain within the provinces they had fought in. Some, such as the vexillatio equitum Illyricorum, even became fully functioning units in their own right.

A typical combat detachment would normally be composed of one or more cohorts (c. 480 men) each of which could be separated from its parent legion easily and also take advantage of its internal six-century organisation. These centuries retained their own staff and had the ability to work independently. Together as a cohort the centurions commanding the centuries provided excellent leadership and an officer would be assigned to lead the detachment. Although his title was praepositus, he was most likely one of the five young tribunes (tribuni angusticlavii) that ordinarily performed staff functions within a typical legion. Assignment to lead a detachment could be great opportunity in the career of a young tribune, eager to make his mark.

Marcus Aurelius, the philosopher emperor, struggled to field enough soldiers using the system of vexillations alone. He was forced to create three new legions, Legio I, II and III Italica, but as tradition dictated, two of these were then settled into legionary bases on the frontier. Within five years these new legions were themselves sending out vexillations to places like Salonae, where detachments helped fortify the city against barbarian attacks.

There was no other easy way to mobilise troops for some new and troubling crisis. An emperor could either move entire legions, gambling that the frontier they protected would remain peaceful in their absence, or he could call on numerous individual detachments, which sometimes spread a legion out across more than one continent. During the crisis years of the third century attacks on the frontier became simultaneously more frequent and more widespread, the constant need for quick reaction forces necessitated the use of the vexillation. It could be mobilised rapidly and arrive at the frontier hot-spot to fight alongside other vexillations under a temporary commander. During the early third century vexillations sent to garrison frontier forts might expect to be deployed for up to three years. For those detachments engaged in field operations, however, a return home might be many more years away. Warfare in this chaotic century was almost constant and a full-strength vexillation was always needed somewhere along the frontier. A number of detachments spent so long operating in the field that they became, in effect, independent combat units.

Did Severus have an answer? He certainly moved legions around to help leverage the manpower he needed for his attacks on Parthia in 197 (and later in 208 on the north of Britannia). He also made use of vexillations to supplement his forces. Did his newly created Legio II Parthica, based in Italy, constitute a new type of military reserve? Although there is no evidence that Severus mobilised its troops to fight on the frontiers, his son, Antoninus, certainly did take large numbers of the legion eastwards to battle against the Parthians.

The New Reserve

The first Christian emperor of the Roman Empire, Constantine the Great, had at his command a mobile field army supplemented by entrenched frontier forces, or limitanei. While the limitanei garrisons slowed an enemy invasion but did not stop it, the field army was tasked with rushing to the region to prevent the barbarian force from penetrating any deeper into imperial territory. This was defence ‘in depth’ that planned to catch the enemy after it had crossed the frontier. It was the way of the future and a new system of military organisation that would be matched against the almost overwhelming barbarian invasions of the fourth and fifth centuries. Although Constantine ruled more than a hundred years after the time of Septimius Severus, it is possible to see the very start of this revolutionary new concept within Severus’ own strategy.

Mobile armies, independent of fixed legions, are often seen as a feature of the Late Roman era, but the historian Michael Speidel noted that ‘the field army is, in a sense, as old as the units stationed in Rome.’ The Praetorian Guard had been Rome’s garrison during the first two centuries of imperial rule but it was rarely deployed to a battle-front. Septimius Severus changed all that, in effect turning the Guard into Rome’s first mobile (or ‘imperial’) field army. As we saw in chapter one, the emperor opened up recruitment to veterans of his Pannonian legions as a reward for loyal service. This also had the effect of elevating the Guard to the status of an elite fighting unit; members were all now battle-hardened veterans. One Praetorian proudly proclaimed on his tombstone that ‘he had served in all the expeditions’.4

At a standing size of 10,000 soldiers and without the onerous administrative duties of units on the frontier, the Praetorian Guard had become the largest combat-ready force within the empire. It remained so throughout the third century and when paired with the new Severan unit, Legio II Parthica, became what was essentially the first effective imperial field army. The II Parthica was a regularly-sized legion of between 5,000 and 6,000 troops, but like the Guard, had no other duties. It was a lean fighting unit with an effective manpower greater than many other legions. Without other duties to tie it up, the II Parthica was always ready to march and to fight. Since it was always available, the II Parthica became the personal legion of the third century emperors and the unit’s commander even became a member of the imperial retinue. In that sense, then, it was not a true independent field army but instead an imperial fighting unit that could provide a reserve of troops for other legions if necessary.

There were a number of other smaller units available at Rome that added to this new reserve, including one of the six Urban Cohorts of military police that patrolled the streets of the capital. Severus increased their membership which meant that a single cohort could contribute 1,500 soldiers to the army reserve. We know at least one cohort could be spared for combat duties after the presence of Urban Cohort XIV was recorded at Apamea in Syria, where it had fought alongside II Parthica.

A number of mounted units had been garrisoned in or around Rome for some time and these may also have bolstered the strength of the Severan army. Foremost among these cavalry units was the imperial guard cavalry, the equites singulares Augusti. Just as he had with the Urban Cohorts, Severus increased the membership of this elite force, doubling it from 1,000 to 2,000. To these he probably added many of the Moorish cavalry that had surrendered to Severus after the battle of Issus in 194. Together with the Mauri auxiliaries that already garrisoned Rome, this light cavalry force numbered around 2,000 individuals and it fought in battle with Antoninus, the eldest son of Severus who was to succeed him. Finally, the ancient writer Herodian recorded that Abgar IX, king of the defeated eastern kingdom Osrhoene, supplied Severus with a force of (horse?) archers for his invasion of Parthia in 197. These cavalrymen then joined the new imperial reserve and may have been garrisoned at Rome within the castra peregrina.5

Amassed together, the Severan reserve may have totalled around 21,500 soldiers, which provided Severus and his successors in the third century with an unprecedented combat force. Vexillations from other legions would then attach to this military core in order to create an expeditionary army able to invade Parthia or tackle the hostile German tribes on their own territory. The variation in troops or detachments assigned to the army depended of course on the location of the threat and the availability of manpower. Emperor Maximinus, for example, raised a unit of German cavalry whilst on campaign in the north, in order to augment his expeditionary force.

Cavalry was proving increasingly valuable. In wars of the past, emperors had time to assemble a large army by marching legions to a troubled province or weak frontier. Once the troops were assembled the invasion would be launched. Often this invasion was either one of retribution for some recent enemy attack, or more likely a pre-emptive strike against a rising power. Rome no longer had the time for this kind of strategy. What the third century crisis needed was mobility and a rapidly reacting force that could attempt to deal with constant raids and invasions on many different frontiers simultaneously. Having an army almost permanently in the field led personally by the emperor addressed this new demand, as did a recognition of the value of cavalry.

Soldier emperors had to lead from the front, which meant that Rome saw less and less of them. Meanwhile, well-fortified frontier cities that lay on good communication routes behind the embattled frontiers began to serve as ad hoc imperial centres. Colonia, Treverorum, Aquileia, Sirmium, Mediolanum, Vindobona … all saw as much, if not more, of the emperor and his retinue than did Rome. Emperors could not leave this powerful military force in the hands of a trusted officer. This was to become a factor in any military deployment that had been relatively uncommon prior to the third century. Back in the first century AD the emperor Claudius, for example, had been able to leave the conquest of Britain to his general Aulus Plautius without fear that the general might suddenly turn the invasion force toward Rome.

The third century would illustrate time and time again that there were no longer any trusted men out there. A cycle of civil wars spanned the century as a long line of usurpers rose to seize the throne for themselves. Many emperors died violently and most of those deaths were the result of assassination. However, it was easier to gain power in the third century than it was to hold on to it; the reign of most of these emperors lasted for no more than a few months. The year 253, for example, saw the emperor Trebonianus Gallus murdered by his own troops as he prepared for battle against an usurper, Aemilius Aemilianus. Within months one of Aemilianus’ own generals, Valerian, declared himself emperor and marched his armies south to seize the throne. Confrontation was avoided when Aemilianus’ own army lynched him near Spoleto in October. While Gallus had reigned for just over two years, the rule of Aemilianus had lasted only eighty-eight days.